In the past, cathode-ray tube display devices were the mainstream of display devices, but in recent years the needs for thin flat-panel displays (FPDs) have been growing. There are various kinds of FPDs, among which known are non-self-luminous liquid crystal displays (LCD), self-luminous plasma display panels (PDP), inorganic electroluminescent (inorganic EL) displays, organic electroluminescent (organic EL) displays, for example.
In particular, organic EL displays have thin, lightweight display-use elements (organic EL element) and have properties such as low-voltage driving, high luminance, and self-luminance. Because of this, research and development of the organic EL displays have been actively carried out. Recently, there have been expected applications of the organic EL displays to light sources for an electrophotographic copier, a printer, an organic semiconductor laser, and other devices and to luminous objects such as a display, lighting, a liquid crystal backlight, and the like. The use of the organic EL elements, which provide surface emission, as luminous objects offers the advantages of showing high color rendering properties and facilitating light controls. Further, the organic EL elements have many advantages. For example, the organic EL elements are mercury-free, while fluorescent lamps contain mercury. Besides, the organic EL elements emit ultraviolet-free light.
Typical organic EL elements are heterojunction-type organic EL elements as shown in FIG. 5. FIG. 5 is a diagram showing a cross-section of a conventional heterojunction-type organic EL element 120a. The heterojunction-type organic EL element 120a shown in FIG. 5 has a multilayer structure wherein a hole injection layer 113, a hole transport layer 114, a light-emitting layer 115, a hole blocking layer 116, and an electron transport layer 117, an electron injection layer 118, and other layers are provided between an anode 112 and a cathode 119. The organic EL element employing such a multilayer structure enables increase in luminous efficiency and increase in lifetime of light emission.
In recent years, organic EL elements prepared with a phosphorescent material as a light-emitting layer have been spreading. The organic EL elements prepared with a phosphorescent material have the advantages of a high luminous efficiency and a long lifetime of light emission.
The organic EL element having the above-described structure enhances luminous efficiency and lifetime of light emission. However, a complex layer structure of the organic EL element results in the problem of a complicated manufacturing process, thus causing increase in cost of a manufacturing apparatus, increase in material costs, and other problems.
In order to solve the above problems, a homojunciton-type organic EL element composed of a single matrix which includes a light-emitting region between the electrodes has been studied. However, it is not easy to achieve an organic EL element with a high performance of a host material and a high efficiency in injection of holes and electrons from an anode and a cathode. As a result, there have been developed a homojunction-type organic EL element 220b doped with a material that enhances an efficiency in injection of holes and electrons, as shown in FIG. 6.
For the homojunction-type organic EL element 220b shown in FIG. 6, a matrix (host) of a single substance is doped with other substance from outside, thereby forming an acceptor region 200, and a light-emitting region 201, a donor region 202, and others, all of which constitutes a positive and negative charge transporting light-emitting layer. That is, the homojunction-type organic EL element has a plurality of regions in the single matrix (positive and negative charge transporting light-emitting layer). Such a structure, which is a simple layer structure, enables a simplified manufacturing process.
For example, Non-Patent Literature 1 discloses a homojunction-type organic EL element emitting light beams of three primary colors. The organic EL element disclosed in Non-Patent Literature 1 is such that an organic thin film of 50 nm to 100 nm in thickness and an aluminum (Al) metal (cathode) are sequentially formed on a transparent electrode (anode) of indium tin oxide (ITO) by vacuum deposition. The organic thin film contains bis(carbazolyl)benzodifuran (CzBDF) as a single matrix (host). An area of 30 nm from the anode in the matrix is subjected to p-type doping by co-deposition with an inorganic oxidizing agent (vanadium pentoxide). On the other hand, an area of 20 nm from the cathode is subjected to n-type doping by the co-deposition with a reducing agent (metal cesium). The above structure facilitates injection of charges from the electrodes to CzBDF and transport of charges.
In addition, in the organic EL element of the Non-Patent Literature 1, an intermediate layer (50 nm to 100 nm in thickness), which is not doped with an oxidizing agent and a reducing agent, is doped with a blue fluorescent pigment, a green fluorescent pigment, and a red phosphorescent pigment. This realizes primary-color light emission attributed to light emission of the pigments. In particular, a green fluorescent element exerts an external quantum efficiency of 4.2%, which is close to a theoretical efficiency limit (5%) of a fluorescent organic EL element, at a high luminance of 60,000 candelas/m2.
The properties described above are considered to be attributed to the following CzBDF characteristics (1) through (3):
(1) being highly balanced, of high mobility, and capable of transporting positive and negative charges;
(2) being of an adequately large energy gap (on the order of 3 eV) between a highest occupied molecular orbital (HOMO) and a lowest unoccupied molecular orbital (LUMO), and being a wide gap material; and
(3) being capable of effectively trapping electrical charges in luminescent pigments.